6.10 Light steel framed walls and floors

6.10.18 Air and vapour control layers

Air and vapour control layers (AVCLs) shall restrict the passage of vapour from within the home to the steel frame and be correctly installed.

An AVCL should be provided, unless a condensation risk analysis shows it is not necessary. An analysis in accordance with BS EN ISO 13788 (Glaser method), using the following boundary conditions, will generally be acceptable:

  • >60% internal RH
  • at 21ºC internally
  • at -2ºC externally.

Split layers of AVCL-type material should be avoided, except where condensation risk analysis shows it to be acceptable.

Where they are provided, AVCLs should be:

  • 500g polyethylene sheet, vapour control plasterboard, or material assessed in accordance with Technical Requirement R3
  • fixed on the warm side of the wall insulation and frame
  • in accordance with the design
  • placed to cover the external wall, including base rails, head rails, studs, lintels and window reveals
  • overlapping the base rail
  • fully sealed and punctures made good.

Where polyethylene sheet is used:

  • each joint in the AVCL should be located on studs or noggings and lapped by a minimum of 100mm
  • double-sided tape or adhesive should be used as a temporary fixing before the wall board is fixed.

Where vapour control plasterboard is used:

  • joints between sheets should be positioned on studs or noggings
  • care should be taken not to displace the vapour control material when cutting vapour control plasterboard.

Last updated: 2nd January 2024

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